Spectrums used in a wireless communications system are classified into a licensed spectrum and an unlicensed spectrum. For a licensed spectrum, generally, after a license is obtained, a corresponding licensed carrier may be used to provide a related communication service, and no resource competition problem exists. For example, in a Long Term Evolution (LTE) system, at a transmit end, each subsequent data frame is sent in sequence if an initial sending time of a data frame is determined. Accordingly, at a receive end, if the initial sending time of the data frame is acquired using a synchronization signal, a receiving time of each subsequent data frame is determined, and the receive end does not need to adjust the receiving time of each frame. For an unlicensed spectrum, anyone is authorized to use a corresponding unlicensed carrier to perform a communication service. However, for data transmission using an unlicensed carrier, data transmission starts only when a time resource is obtained by means of time-based competition, and after a period of time of transmission, sending must be stopped and a channel is released such that another device has an opportunity to occupy the channel.
Therefore, for an unlicensed carrier, the resource competition problem exists, and the transmit end cannot occupy the channel all the time to transmit data. Therefore, a moment at which data is sent varies with a moment at which a resource is successfully obtained by competition, and therefore, an original receiving mechanism for a licensed carrier is unsuitable to use for an unlicensed carrier. Currently, for a communications system that uses an unlicensed carrier, a receive end needs to detect a channel of the unlicensed carrier all the time. A biggest problem of this solution is heavy power consumption, which affects standby time of the receive end.